1. Field of the Invention
This invention relates generally to semiconductor devices, and more specifically, to ohmic contact structures for float zone silicon wafers and for silicon carbide wafers and to methods for fabricating such contact structures.
2. Description of Related Art
It is well known to form a semiconductor device using a semiconductor die that includes a N− epitaxial silicon layer formed atop a N+ float zone silicon substrate, for example, the device being formed in the surface of the epitaxial layer and electrode contact metals being formed on the upper and back surfaces of the die. For example, a fast recovery diode (FRED) generally includes a plurality of PN junctions formed in the upper portion of the epitaxial layer and spaced by Schottky regions. A common top electrode is shared by the PN junctions and Schottky regions to form a device that includes PN and Schottky diodes. A back electrode is formed along the back surface of the N+ float zone silicon substrate. The need to grow the epitaxial layer on a semiconductor die, however, makes the die expensive, whether used for a FRED or any other device.
Accordingly, it would be desirable to form a device directly in/on the surface of a float zone silicon die that does not have the epitaxial layer. Once forming device components in/on the upper surface of the die, a back surface contact metal must thereafter be formed. In a conventional device, the presence of a high conductivity region at the back surface of the die (e.g., N+ or P+ region) lowers the contact resistance between the contact metal and the silicon body, thereby resulting in a good ohmic contact. However, because a float zone die is usually lightly doped (e.g., N− or P−) it does not form a good ohmic contact with a contact metal.
Notably, an implant can be performed along the back surface of the die to improve its capability of forming a good ohmic contact. Such a step, however, requires additional handling, which may be undesirable when an ultrathin device is desired. In addition, an implant may not be feasible when an ultrathin device is desired.
Similarly, it is known to form a device in the upper surface of a semiconductor die made of silicon carbide. Here again, a back side contact metal needs to be formed such that the contact metal makes good ohmic contact to the silicon carbide. However, the formation of such ohmic contacts has also been quite difficult to form, as described in U.S. Pat. No. 5,980,285. For example, it has been difficult to sufficiently dope the silicon carbide surface to form an N+ region for contact by a contact metal.